https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95237
--- Comment #13 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 2 Jun 2020, hjl.tools at gmail dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95237 > > --- Comment #12 from H.J. Lu <hjl.tools at gmail dot com> --- > (In reply to H.J. Lu from comment #10) > > (In reply to rguent...@suse.de from comment #9) > > > > > > The i386 psABI specifies 4 byte alignment for long long. But we want to > > > > use 8 byte alignment if there is no ABI implication and no stack > > > > realignment > > > > is needed. Will ROUND_TYPE_ALIGN always align long long to 8 bytes? > > > > > > So what we want here is RTL expansion pad out stack slots if they know > > > to be aligned but not adjust DECL_ALIGN. Not sure if there's already > > > a target hook to pad out variables, if not this may be the way to get > > > what you desire here. > > > > > > I guess the actual reason is crossing of cache-lines? > > > > If the incoming stack is 4 byte aligned, we can't align long long to > > 8 bytes without stack realignment. > > Only for local variable on stack. We still want to align global/static long > long to 8 bytes. Sure. And yes, ROUND_TYPE_ALIGN doesn't work for that since it only gets the type and not the decl. We'd need a ROUND_DECL_ALIGN for this. I alternatively suggested that LOCAL_DECL_ALIGNMENT should be used there and I guess we can do that for alignment lowering.
